PROJECT SUMMARY Pain management poses significant clinical challenges due to its very high prevalence, the diversity of patients and underlying mechanisms, and the lack of safe and effective treatment options. Approximately 50 million Americans experience acute post-operative pain, whereas about 100 million US adults suffer from chronic pain. Opioids remain the most effective class of analgesics for moderate-to-severe pain, and are thus broadly used. Their analgesic activity is mediated through activation of mu-opioid receptors (MOR). Unfortunately, MOR activation also results in serious adverse effects, especially respiratory depression, a primary cause of death in addict overdoses as well as in patients with compromised respiratory and kidney function or who self-medicate with other drugs (e.g., tranquilizers, alcohol). Other serious generally non-lethal adverse effects include mental clouding and somnolence, addiction, physical dependence, and constipation. Despite these concerns and for the lack of better options, the number of opioid prescriptions, dose per prescription, and number of days of treatment have increased in recent years. This has led to a ?prescription opioid epidemic? resulting in unprecedented levels of addiction and overdose related deaths. Peripherally-restricted peptidic kappa-opioid receptor (KOR) agonists are emerging as a new class of non-addictive and safe analgesics. CR845, a first-in- class all D-amino acid tetrapeptide KOR agonist with high potency and selectivity for the KOR and lack of brain penetration, has established in clinical trials that peptidic KOR agonists (i) produce analgesia, (ii) lack central adverse effects, (iii) do not produce constipation or respiratory depression, and (iv) reduce the need for MOR agonist supplemental therapy in post-operative pain. CR845 is however short-acting, requiring multiple daily intravenous dosing, thus limiting its clinical utility and commercial interest. Under a previous SBIR Phase I award, we have established the feasibility to conjugate the above tetrapeptide KOR agonists to CVX-2000, a proprietary monoclonal antibody carrier previously used and clinically-validated to create long-acting peptide-antibody conjugates (PACs) enabling once-weekly subcutaneous dosing. Our current long-acting KOR (LA-KOR) agonist lead series has achieved potent KOR agonist activity and selectivity, and demonstrated preclinical analgesic activity. The proposed follow up SBIR Phase II program seeks to: Aim 1: further optimize the existing lead LA- KOR agonist series allowing selection of a LA-KOR agonist clinical candidate for development; Aim 2: characterize the antinociceptive profile of LA-KOR agonists in a broad range of disease-relevant pain models, in both male and female animals, to validate clinical indications; Aim 3: complete key preliminary preclinical studies to help design investigational new drug (IND)-enabling studies. IMPACT & Relevance to Public Health: If successful, this program will bring forward novel, efficacious, non-addictive, safe, and convenient analgesics able to reduce or replace opioids for the treatment of moderate-to-severe pain, thus improving pain control while maintaining patient safety and quality of life, and helping society to address the opioid crisis.